## Supercapacitors

Bridging the gap between electrochemical batteries and capacitors.

### Background

Supercapacitors (also called an Electric double-layer capacitor) or Ultracapacitors are a type of high power high energy density capacitor. Whereas a conventional capacitor uses a solid dielectric to separate the capacitor plates, a super capacitor consists of two metal electrodes coated with a high surface area type of activated carbon and separated by a thin porous insulator all in an electrolyte. The electrolyte depends on the application, the power requirement or peak current demand, the operating voltage and the allowable temperature range and can be either aqueous or non-aqueous. They can store or deliver energy at a very high rates. The energy stored by a capacitor, E (Joules), is given by:

$E = \frac{1}{2} C V^2$ (1)

Where $E$ is the electrostatic energy stored, $C$ is the capacitance and $V$ is the voltage difference across the capacitor plates. The capacitance C is given by:

$C = \epsilon_r \epsilon_0 \frac{A}{d}$ (2)

The very large surface area of the porous electrode allows for much higher capacitance than conventional capacitors.

Figure: illustration of a supercapacitor (see Enerize)

### Characteristics and Applications

The energy density of supercapacitors is generally an order of magnitude less than that of conventional batteries, but the power density is generally 1-2 orders of magnitude greater. They can also be cycled much more effectively, generally showing very little degradation over thousands of charge/discharge cycles [1]. They suffer from relatively high self-discharge rates compared to conventional batteries and require more complex electronic control as the voltage of capacitors falls significantly as they are discharged. Most current applications rely on combining super capacitor and battery technology, in order to combine the power performance of the former with the energy storage capability of the latter. Commercially super capacitors are available with energy densities of 5 Wh/kg, although experimental variants exist based on graphene materials with 28.5 Wh/kg and a power density of 10k W/kg [2, 3].

Supercapacitors have potential applications in EV’s as they can absorb power fast enough to be used for regenerative breaking. This is a major drive in development of grapheme supercapacitors. They also have potential applications for PV integration as they can be used to alleviate voltage swings from large PV-installations that otherwise pose operational issues on the grid. South Korea has significant expertise in supercapacitors, both in academic sectors and commercial (NESSCAP).

### Summary of characteristics

 Typical Capacity Typical Power Efficiency (%) Storage Duration $/kWh$/kW Lifespan Cycling capacity Up to 1 kWh Up to 300 kW >90 [4]95 [5] Secs - mins 500 – 1000 [4], 10000 [5] 200 - 400 [4], 500 [5] 20 years High

Table: Supercapacitor characteristics